The disposal of low-value carbon-containing waste materials is a major challenge and problem. This is particularly true for carbon-containing waste materials that have high ash content, are in chunks, and/or are wet. Examples of these materials include wet biosolids (sludge) or dry biosolids from municipal wastewater plants, municipal solid waste (MSW), rice hulls, biomass such as log-yard waste and forestry waste, and mixed or separated rubber, plastic, and/or paper.
Table A below lists the approximate values of some relevant properties for some of these example carbon-containing waste materials.
TABLE AWaterAsh DisposalAvail. contentcontentcostenergyMaterial(%) (%)Chunks($/ton)(BTU/#)Wet biosolids826-7No  45-70+N/A(sludge)Dried biosolids,1030+No40-506,80020% of wet weightMSW3514Yes27-504,200Rice hulls1510No 0-255,600Log-yard waste258Yes10-275,200(biomass)Mixed rubber/128Yes27-4010,000plastic/paperAs can be seen from this table, all of these example carbon-containing waste materials tend to have relatively high ash content. All of these example materials except rice hulls and biosolids have significant chunks in the material, even when shredded. And wet biosolids and forestry waste biomass as processed include very large amounts of water.
For disposal, these materials have traditionally been incinerated, land-filled, or left to biodegrade, with the energy value contained in them left unrecovered. But because of their high water content, high ash content, and/or chunky consistency, disposing of these materials can be a real challenge. Materials with high water content, such as wet biosolids and forestry waste biomass, tend to be high in weight and volume, and this increases the cost to transport them to a site for incinerating, land-filling, or biodegrading. So these materials are typically dried by natural-gas-powered dryers to reduce their weight and volume. But this requires burning additional fuel, adding to the cost and pollution issues. And wet biosolids in particular cannot be incinerated without prior drying unless additional fuel is burned during incineration. For materials with high ash content, the ash typically becomes sticky during incineration by temperatures of about 1400 F, thereby requiring frequent processing-system shut-downs to clean out the equipment. And materials that have chunks are not easily fed continuously and uniformly into the process for combustion. Additionally, chunky materials must have considerable residence time at combustion temperature to burn until consumed, and chunky materials with high ash content tend to form surface coatings of ash that inhibit complete burnout unless they are mechanically agitated. Furthermore, generally speaking, incineration and land-filling solutions have major public-perception problems.
Another option for disposal of carbon-containing waste materials is gasification. This approach typically recovers some energy from the material (in the form of producer-gas fuel aka syngas) and as such is more environmentally acceptable. And this approach achieves an initial separation of the ash at a lower temperature (typically about 100 F lower) and in less time, with minimal slagging of the ash as there are less volatile toxic metals (e.g., Pb & As) in the vent gases and therefore less air pollution. But this approach also has its drawbacks. For example, for waste materials having high ash content, the ash sometimes becomes sticky by temperatures lower than 1400 F under reducing conditions during gasification.
Accordingly, it can be seen that there exists a need for a better environmentally acceptable way of disposing of carbon-containing waste materials that have high ash content, are in chunks, and/or are wet. It is to the provision of solutions to this and other problems that the present invention is primarily directed.